If a response callback gets called with a nil response value, then …

…replace that value with `[NSNull null]` instead of `@{}`. This way the javascript will receive a `null` value instead of `{}`. See gh pr marcuswestin#73

WebViewJavascriptBridge/WebViewJavascriptBridge.m

@@ -175,7 +175,7 @@ - (void)_flushMessageQueue {
             if (callbackId) {
                 responseCallback = ^(id responseData) {
                     if (responseData == nil) {
-                        responseData = @{};
+                        responseData = [NSNull null];
                     }
 
                     WVJBMessage* msg = @{ @"responseId":callbackId, @"responseData":responseData };